The shooter fired about six shots from inside the school toward officers as they approached outside, Alexander said, and police returned fire.

Michael Brandon Hill is charged with aggravated assault on a police officer, terroristic threats and possession of a firearm by a convicted felon, police said.

Police said it is unclear whether the suspect had any connection with the school, the Ronald E. McNair Discovery Learning Academy, which is about seven miles east of downtown Atlanta. Investigators think the gunman entered the school behind someone, Alexander said.

As the standoff unfolded, a caller told CNN affiliate WSB's assignment desk that she was in the school office with a gunman.

The man had a message he wanted the woman to share with the local television station, assignment editor Lacey LeCroy said, describing the conversation during WSB's newscast Tuesday evening.

"She said he told her to tell me he was not afraid to die," LeCroy said.

The gunman also threatened to harm police, the woman said, according to LeCroy.

Later, LeCroy heard gunshots crack over the television station's phone line. The woman in the school office wasn't sure who had fired, LeCroy said.

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Police: Gunman carried AK-47 into school

Shots fired at Georgia school – A policeman walks in front of Ronald E. McNair Discovery Learning Academy after a shooting incident in Decatur, Georgia, on Tuesday, August 20. Michael Brandon Hill, 20, opened fire at the school armed with an AK-47 "and a number of other weapons," police said. There were no reports of injuries.

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Shots fired at Georgia school – A police officer interacts with students as they board school buses to take them to reunite with their parents.

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Shots fired at Georgia school – A child peers out the window of a school bus as the children are returned to their parents at a nearby shopping center Tuesday afternoon.

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Shots fired at Georgia school – Dekalb County Police SWAT officers run though the front yard of a home toward the elementary school on Tuesday.

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Shots fired at Georgia school – A police officer stands at the scene after the shooting. The school is located about seven miles east of downtown Atlanta.

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Shots fired at Georgia school – Nicole Webb gets emotional while waiting for her 9-year-old son.

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Shots fired at Georgia school – Parents cheer as buses arrive with students on board.

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Suspected school shooter in custody

Investigators initially suspected there could be explosives in a vehicle the shooter parked outside the school, but police found no explosives inside, Parish said.

Teachers and administrators guided students out to a lawn outside the school's gym, where they remained while investigators combed the school to ensure there was no other threat. Concerns about explosives forced SWAT teams to cut a hole in a fence by the school and evacuate students through the yard of a neighboring home, Alexander said. On a nearby street, school buses waited to take them to safety.

"This was a very unusual situation where we had to get the kids away from any possible explosives," Alexander said.

This was the second arrest in six months for Hill, of DeKalb County. In March, he was arrested in neighboring Henry County on suspicion of "terrorist threats and acts," according to a record in the county sheriff's office. He was released on bail five days later. Information on what led to that arrest or whether the case is resolved wasn't immediately available.

Students were reunited with their parents at a nearby shopping center Tuesday afternoon. The children smiled and waved, and parents cheered, as each school bus arrived with students aboard.